Method of driving an emitter array

ABSTRACT

Methods and apparatus for driving an emitter array are described. A method includes determining a thermal environment profile for a plurality of emitters of the emitter array, computing a current pulse profile for at least one of the plurality of emitters based on the thermal environment profile, and applying a current pulse with the computed current pulse profile to the at least one of plurality of emitters.

FIELD OF INVENTION

The invention describes a method of driving an emitter array, and alsodescribes an LED arrangement.

BACKGROUND

Arrays of very small non-encapsulated light-emitting diodes can be usedfor various applications such as camera flash or automotive lightingapplications. Such non-encapsulated light-emitting diodes are generallyreferred to as direct emitters. One or more layers ofwavelength-converting phosphor may be used to obtain a desired outputwavelength desired range. To deliver the desired brightness, uniformity,colour temperature, colour over angle, etc. in such applications, theemitters of an array must be driven with high relative current densitiesduring short pulses. The small emitter size means that relatively highcurrent densities, junction temperatures and phosphor temperatures willbe reached during a pulse. It is known that high temperatures have anadverse effect on diode performance. The extent to which emittertemperature affects its light output depends on factors such as thesystem thermal design, the type of phosphor used, the emitter's hot-coldfactor, etc. Various approaches have been suggested to compensate forthe effects of temperature, but these solutions usually address thespecific problem of how to ensure that a uniform white light can beproduced using a combination of red, green and blue emitted diodes,since the different colour LEDs respond differently to temperaturefluctuations. The prior art documents US2010259182A1, US2010301777A1,US20060237636A1 and US20120306370A1 consider various ways of ensuring ahomogenous light output using different coloured LEDs.

An emitter array of high-power direct emitters may be used to illuminatea scene, for example an automotive front lighting application, a cameraflash module, etc. The effect of temperature on the performance of theemitters can result in undesirable changes in illumination and colourshift. This can be particularly noticeable for a camera flash module,for example when the image sensor is used in conjunction with a rollingshutter technique. The effect can also be noticeable in the case of acolour-tunable flash, because the colour and intensity of the differentemitters will react differently to temperature.

Therefore, it is an object of the invention to provide away of drivingan array of emitters to overcome the problems outlined above.

SUMMARY

The object of the invention is achieved by the method of claim 1 ofdriving an array of emitters; and by the LED arrangement of claim 10.

According to the invention, the method of driving an emitter arraycomprises at least the steps of determining a thermal environmentprofile for the emitter array; computing a current pulse profile for anemitter on the basis of the thermal environment profile; and applying acurrent pulse with the computed current pulse profile to that emitter.

Generally, a current pulse profile for an emitter is computed primarilyon the basis of a required light output parameter, i.e. the colourand/or the required light intensity of that emitter, the desiredbrightness, uniformity, colour temperature, colour over angle, etc. Theinventive method augments this requirement by information relating tothe thermal environment affecting each emitter of the emitter array.

The invention is based on the insight that the performance of an emitterin response to a current pulse is not only determined by the amplitudeof the current pulse, but also by the overall thermal environmentaffecting that emitter. The expression “thermal environment profile” asused in the context of the invention is to be understood as thecollective thermal influences acting on each emitter of the emitterarray—for example the changing temperature of an emitter (following apreceding current pulse that was applied to that emitter and/or to aneighbouring emitter) as well as the changing temperature of anyneighbouring emitters.

When a current pulse is applied to an emitter, the emitter is heated tosome extent. Whereas a current pulse may be assumed generally to be verybrief, the resulting “thermal pulse” is longer and decays relativelyslowly. The thermal effect on an emitter is therefore of a longerduration than the current pulse that caused it. Since flux (change inlight output over time) and colour temperature of an emitter are alsotemperature-dependent, the inventive method allows these emitterproperties to be managed in a controlled manner. The thermal environmentprofile may be assumed to be constantly changing until such time as allthe emitters have the same temperature, for example the ambienttemperature. The thermal environment profile quantifies the continuallychanging temperature development over the entire emitter array. At anypoint following a number of current pulses applied to one or moreemitters, the thermal environment profile might be visualized as a 3Darray of temperature regions corresponding to the array of emitters, sothat the height and orientation of a peak region represents thetemperature distribution over the corresponding emitter. Similarly, theshape of a region and the slopes of its sides may describe thetemperature distribution over the corresponding emitter. A uniform andessentially unchanging thermal environment profile of an emitter arraymay be regarded as a steady state thermal environment profile.

An advantage of the inventive method is that the shape of a currentpulse applied to an emitter will complement the effects of the thermalenvironment acting on that emitter array at that instant. In otherwords, the current pulse applied to an emitter will have been computedto take into account any negative effects arising from the temperatureof that emitter as well as the temperatures of its surrounding emitters,as well as to take into account the required light output, the desiredon-scene colour temperature, etc. For these reasons, the inventivemethod may be regarded as an adaptive driving scheme.

According to the invention, the LED arrangement comprises an array ofemitters; a thermal environment module realised to determine a thermalenvironment profile of the emitter array; a current profile computationmodule realised to compute a current pulse profile for an emitter of theemitter array on the basis of the thermal environment profile; and adriver realised to apply a computed current pulse profile to thecorresponding emitter. The current profile computation module preferablyalso takes into consideration any emitter characteristics that are afunction of temperature, for example light output (flux), colourtemperature, etc.

The inventive LED arrangement can ensure that an emitter array alwaysdelivers a desired light output pattern, for example a homogenous oruniform light output pattern, even if the emitters of an array areunevenly heated. This improved performance can be achieved withrelatively little effort and without any significant alterations incircuit design, so that the inventive LED arrangement can bemanufactured at favourably low cost.

The dependent claims and the following description disclose particularlyadvantageous embodiments and features of the invention. Features of theembodiments may be combined as appropriate. Features described in thecontext of one claim category can apply equally to another claimcategory.

In the following, without restricting the invention in any way, it maybe assumed that an emitter of the array is very small, for example avery small non-encapsulated light-emitting diode (a direct-emittingLED), a vertical-cavity surface-emitting laser (VCSEL) diode, etc. Adirect-emitting LED may be realised to emit white light, or may berealised as an infrared-emitting LED (IR-LED). Preferably, an emitter isrealised as a sub-500 micron LED, i.e. with a light-emitting area of atmost 0.25 mm2. As will be known to the skilled person, such an emittercan be covered in a wavelength converting material such as a phosphor,and may be protected from the environment by a transparent protectivecoating such as silicone, and is not manufactured to include any opticelement. Instead, light-shaping optics can be included as part of thedevice in which the LED will later be mounted.

Preferably, the emitters are arranged very closely together in anemitter array. The emitters of the emitter array may be realised asindividual dies that are mounted very close together onto a commonsubstrate. Alternatively, the emitter array may be realised as amonolithic die with a closely packed array of emitters. In a monolithicdie, the emitters can all be fabricated in a single process using thesame materials and having the same layer structure and therefore alsoessentially identical colour points.

The emitter array in an embodiment of the inventive arrangement cancomprise any number of emitters arranged in an array, for example nineemitters arranged in a three-by-three array. Essentially, there is nolimit to the size of such an emitter array, so that even very largearrays in the order of 100×100 emitters or more are possible, forexample in a micro-LED display.

In a preferred embodiment of the invention, the LED arrangement isrealised as a camera flash module. In such a realisation, the emitterarray preferably comprises at least one cool white emitter and at leastone warm white emitter so that the colour point of the light can betuned as required to optimally illuminate a scene. Thecurrent/temperature behaviour of a warm white emitter is usuallydifferent than the current/temperature behaviour of a cold whiteemitter. The inventive method or adaptive driving scheme can provide asolution to the problem of how to prevent a pre-flash from affectingimage quality. A pre-flash event is a low-current flash that precedesthe actual high-current flash using during image capture. The pre-flashevent is used to set up the camera, but it can also affect the imagequality because the emitters have been heated immediately prior to thehigh-current flash.

In another preferred embodiment of the invention, the LED arrangement isrealised as an automotive lighting module, for example for daytimerunning lights (DRL), indicator lights, etc. The LED arrangement canalso be realised in a face recognition module. In such an embodiment,the inventive method can be particularly advantageous, since the lightoutput of infrared emitters used in face recognition applications arevery temperature dependent.

As explained above, the thermal environment profile takes into accountthe collective thermal influences affecting each emitter of the emitterarray. Initially, before any current pulse has been applied to anemitter, a thermal environment profile may be considered to be in a“null” state or steady state, since all emitters will have the sametemperature (e.g. the ambient temperature). An initial state of thethermal environment profile may be obtained by measuring the forwardvoltages of the emitters and using this information to estimate thetemperatures of the individual emitters. Alternatively, the emittertemperatures could be measured using a suitable arrangement of negativetemperature coefficient resistors, infrared sensors, etc.

As soon as one or more emitters have been turned on by a current pulse,the thermal environment profile will be in a state of flux until suchtime as a steady-state temperature distribution is reached. In anemitter array that is operated for a relatively long time, for examplein a DRL application, a steady state temperature distribution may bereached after the first few current pulses. In the case of a pulsedapplication such as a camera flash, a steady-state temperaturedistribution will generally only be reached some time after the lastflash event, i.e. the temperature distribution will slowly decay to theuniform ambient temperature when all emitters are “off”.

There are various ways of establishing a thermal environment profile tocomprise information relating to temperature development in the emittersand to describe the temperature development of each emitter. Forexample, a direct feedback approach may be used, based on continuallysampling the forward voltages of the emitters to estimate the emittertemperatures. Such an approach may use a look-up table or calibrationtable to relate forward voltages to emitter temperatures. An alternativeapproach can make use of the near-linear relationship between forwardvoltage and junction temperature.

In a particularly preferred embodiment of the invention, the thermalenvironment profile is established using a thermal predictive model, anddescribes the temperature development of each emitter by a mathematicalfunction. In the inventive method, the thermal predictive model relatesemitter power (emitter current×forward voltage) to emitter temperatureover time. Using the thermal predictive model, the colour temperature ofpart of the scene, illuminated by a certain emitter, can be derived as afunction of emitter current and time. By inversing the relationship, acurrent profile can be calculated for that emitter. By doing this forall emitters of the array, a current profile matrix can be establishedfor a subsequent pulse event. Alternatively, it is possible to model theemitter behaviour, by expressing light output as a function of currentand temperature.

The invention is based on the insight that the temperature of an emitterbehaves in a manner similar to the voltage step response of an RCcircuit. The thermal behaviour of an emitter can therefore be describedby an appropriate thermal time constant. The thermal time constant of anemitter can be established during the manufacturing process. In oneapproach, the temperature response of the individual emitters within acertain application can be determined, so that characteristic thermaltime constants may be derived for the product or design. For example,the temperature response of the emitters of a certain 10×10 array can bedetermined, and characteristic thermal time constants are then derivedfor a camera flash using that type of 10×10 array.

Alternatively or in addition, the thermal time constant of an emittermay be determined numerically. The temperature development of eachemitter is therefore described by a corresponding mathematical function.The thermal predictive model may be regarded as the set of mathematicalfunctions used to describe the thermal behaviour of the emitter array.With this information, the thermal environment profile may be visualizedat any point in time as a dynamically changing array of peaks, wherebythe height of each peak is either increasing (during a current pulse) ordecreasing (following a current pulse). The rate of increase/decrease ofa peak is determined by the thermal time constant of the correspondingemitter and parameters relating to the product design. Similarly, theshape of a peak may morph towards neighbouring, higher peaks. This isbecause a hot emitter will heat its neighbouring emitters. Thisinformation can be used to predict the future behaviour of the emittersfollowing any current pulse pattern and to determine any correctionsthat should be undertaken to the current pulses in order to achieve adesired light output pattern from the emitter array. For example,knowing that a particular emitter will have been heated directly by acurrent pulse and also indirectly from two adjacent emitters, a tailoredcurrent pulse profile may be computed for that emitter in order tocompensate for the lower performance that would otherwise result becauseof the emitter's temperature. The thermal predictive model can predictthe temperature of each emitter at any time, regardless of whether ornot that emitter and/or any of its neighbours have been driven by acurrent pulse. The inventive method may make use of a full mathematicalmodel by first setting up a generic mathematical model and then makingit product-specific by using measured or calculated thermal properties.

Preferably, the thermal environment profile also takes into account thepulse history of each emitter. Depending on the pulse rate used to drivethe emitters, it may be that the temperature of an emitter is still“decaying”, i.e. decreasing, when a subsequent pulse is applied to thatemitter or to one of its neighbours. The residual heat from the previouspulse will have a cumulative effect, so that a greater temperature isreached during the subsequent pulse. Therefore, in a particularlypreferred embodiment of the invention, the (recent) pulse history of anemitter is considered when computing the shape of a subsequent currentpulse. Relevant information to compute a suitable subsequent currentpulse shape may be the thermal time constants of the neighbouringemitters, a thermal time constant of a sub mount or PCB, etc. A suitablemodel can be used to compute the shape of a subsequent current pulse.

When a phosphor layer is applied to an emitter to perform wavelengthconversion, the colour temperature of the emitter can change as afunction of the emitter temperature due to spectral blue pump shift anda temperature-related change in phosphor behaviour. In an emitterwithout a wavelength-converting phosphor layer, an increase intemperature may result in a spectral shift. Therefore, in a preferredembodiment of the invention, the thermal predictive model also considersthe temperature dependency of a desired colour point. This can be doneby deriving a temperature-dependent behaviour model of the emitters onthe basis of measurements, for example. When using warm white and coldwhite emitters in a single array, the on-scene colour temperature can bekept stable by adjusting the warm-white/cold-white flux ratio during thepulse.

The current pulse profiles for the emitters can be computed to adjustthe ratio of a light output parameter of a first emitter to thecorresponding light output parameter of a second emitter. A light outputparameter can be any of brightness, colour temperature, etc. This can beadvantageous in a colour-tunable LED arrangement in which the emitterarray comprises at least two LEDs having different colours. For examplean LED flash module can use an emitter array that has at least one coldwhite emitter and one warm white emitter, each with phosphor layers forwavelength conversion. Because an emitter heats up very rapidlyfollowing the start of a current pulse, and because the phosphorbehaviour is temperature-dependent, any difference in colour may be mostpronounced at the start of a pulse event. In a preferred embodiment ofthe invention, therefore, the current pulse profiles for the emittersare computed to dynamically adjust the ratio of the first emitter colourto the second emitter colour during a current pulse. In this way, withrelatively little effort, the ratio of warm white to cool white can beadjusted even during a pulse in order to achieve a uniform flash colourtemperature distribution.

Other objects and features of the present invention will become apparentfrom the following detailed descriptions considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for the purposes of illustration and not asa definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an array of direct-emitting LEDs;

FIG. 2 shows rectangular current pulses to drive three emitters of thearray of FIG. 1 using a prior art approach;

FIG. 3 shows thermal response of the nine emitters of the array of FIG.1 using a prior art approach;

FIG. 4 shows light output of the three driven emitters of the array ofFIG. 1 using a prior art approach;

FIG. 5 shows tailored current pulses to drive three emitters of thearray of FIG. 1 using the inventive method;

FIG. 6 shows thermal response of the nine emitters of the array of FIG.1 using the inventive method;

FIG. 7 shows light output of the three driven emitters of the array ofFIG. 1 using the inventive method;

FIG. 8 shows rectangular current pulses to drive nine emitters of thearray of FIG. 1 using the prior art approach;

FIG. 9 shows thermal response of the nine emitters of the array of FIG.1 using the prior art approach;

FIG. 10 shows light output of the nine driven emitters of the array ofFIG. 1 using the prior art approach;

FIG. 11 shows tailored current pulses to drive nine emitters of thearray of FIG. 1 using the inventive method;

FIG. 12 shows thermal response of the nine emitters of the array of FIG.1 using the inventive method;

FIG. 13 shows light output of the nine driven emitters of the array ofFIG. 1 using the inventive method;

FIG. 14 shows tailored subsequent current pulses to drive nine emittersof the array after a preceding switching event of the array of FIG. 1using the inventive method;

FIG. 15 shows light output as a function of emitter position in thematrix of FIG. 1 when driven using a prior art approach;

FIG. 16 shows an embodiment of the inventive LED arrangement;

FIG. 17 shows a simplified block diagram of an embodiment of theinventive LED arrangement for driving two different colour arrays;

FIG. 18 shows a further simplified block diagram of an embodiment of theinventive LED arrangement.

In the drawings, like numbers refer to like objects throughout. Objectsin the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows an emitter array 1 with a square arrangement of nineemitters E₁₁, E₁₂, . . . , E₃₃ for use in a camera flash application,for example a flash module of a mobile phone. In this exemplaryembodiment, the emitters E₁₁, E₁₂, . . . , E₃₃ are sub-500 micron dies,i.e. the surface area of each emitter is at most 0.25 mm². The dies areclosely packed and mounted on a common carrier 10 or PCB. The gapbetween any two adjacent emitters may comprise about 100 μm or less.Alternatively, the emitter array 1 could be realised as a monolithicdie. The emitters E₁₁, E₁₂, . . . , E₃₃ are driven individually by adriver module (not shown in the diagram) comprising a driver that issuescurrent pulses for the emitters according to a control algorithm. In thefollowing, graphs of current, temperature and relative light output areshown in a similar 3×3 array layout so that information content of agraph can easily be related to the corresponding emitter.

In the prior art, the driver is realised to drive selected emitters E₁₁,E₁₂, . . . , E₃₃ with identical current pulses. FIG. 2 shows threerectangular current pulses I_(21_pa), I_(22_pa), I_(23_pa) in a timeinterval t1 extending from time 0 to time t₁ to drive the three emittersE₂₁, E₂₂, E₂₃ of the middle row using the prior art approach. The graphsindicate relative current (Y-axis) against time (X-axis). The emittersEn, E₁₂, E₁₃, E₃₁, E₃₂, E₃₃ of the top and bottom rows are to remainoff. The three emitters E₂₁, E₂₂, E₂₃ of the middle row will heat up asa result of their respective current pulses I_(21_pa), I_(22_pa),I_(23_pa), but each of these emitters will also heat its neighbours. Thetemperature development during and after the current pulses is shown inFIG. 3, which indicates relative temperature (Y-axis, in percent)against time (X-axis). Because a hot emitter heats its neighbours, thetemperature T_(22_pa) of the central emitter E₂₂ will rise to thegreatest level. The temperatures T_(21_pa), T_(23_pa) of the two outeremitters E₂₁, E₂₃ of the middle row will also be influenced by thecentral emitter E₂₂, and the emitters En, E₁₂, E₁₃, E₃₁, E₃₂, E₃₃ of thetop and bottom rows—even though they were not turned on—will becomeheated by the middle row of emitters E₂₁, E₂₂, E₂₃. The central emitterE₂₂ is heated by both adjacent emitters E₂₁, E₂₃, and because thecentral emitter E₂₂ is hottest, the middle emitters E₁₂, E₃₂ of the topand bottom rows are also heated more than the outermost four emittersEn, E₁₃, E₃₁, E₃₃.

The temperature of an emitter affects the light output by that emitter.FIG. 4 illustrates this effect, and shows the relative light outputL_(21_pa), L_(22_pa), L_(23_pa) (in percent) by the three emitters E₂₁,E₂₂, E₂₃ of the middle row. The diagram shows that the initially highlight output decreases rapidly as the emitters E₂₁, E₂₂, E₂₃ heat up,and that the relative light output L_(22_pa) of the central emitter E₂₂decreases to the greatest extent. Regarding this light output level as100%, the light output of the outer two emitters E₂₁, E₂₃ decreases to alesser extent, to a level above the 100% mark on the Y-axis. This isbecause the central emitter E₂₂ is also heated the most, being flankedby two hot emitters E₂₁, E₂₃. Instead of delivering a uniform lightoutput pattern, the three emitters driven using the prior art approachwill deliver an uneven light output pattern.

The temperature of an emitter decreases after termination of the currentpulses, but may be still significantly higher than an ambienttemperature by the end of a second time interval after which asubsequent pulse is to be applied to the emitter array.

In the case of an emitter array that implements a combination ofwarm-white and cold-white emitters, the different emitter behavioursover temperature result in deviations from the desired on-scene colourtemperature.

The inventive method provides a solution to these problems and isexplained with the aid of FIGS. 5-7, using the same 3×3 emitter array ofFIG. 1 as a basis. Assuming that the middle row of emitters E₂₁, E₂₂,E₂₃ is to be driven, the approach taken by the invention is to predictthe temperature environment that will affect the relevant emitters, andto compute the current pulse shapes that will be needed to counteractthe negative effects of emitter temperatures. An example of threetailored current pulses I₂₁, I₂₂, I₂₃ is shown in FIG. 5. Instead of thesimple rectangular shapes shown in FIG. 2, the three current pulses I₂₁,I₂₂, I₂₃ applied to the middle row of emitters E₂₁, E₂₂, E₂₃ have shapesor profiles that will ensure a uniform light output even though theemitter temperatures will change during the current pulse event overtime interval t1 (again, the pulse event starts at time 0 and ends attime t₁).

FIG. 6 shows the temperature development in the emitter array followingthe current pulses I₂₁, I₂₂, I₂₃ shown in FIG. 5. The relativetemperatures T₁₁, . . . , T₃₃ of the emitters E₂₁, E₂₂, E₂₃ aredifferent from those shown in FIG. 3 owing to the different shapes ofthe current pulses I₂₁, I₂₂, I₂₃ applied to the middle emitter row.

FIG. 7 shows the relative light output L₂₁, L₂₂, L₂₃ (in percent) of thethree emitters E₂₁, E₂₂, E₂₃ of the middle row after receiving thecurrent pulses I₂₁, I₂₂, I₂₃ shown in FIG. 5. The diagram shows that theinitially high light output (close to 100%) is essentially maintained,even as the emitters E₂₁, E₂₂, E₂₃ heat up, and that the light outputL₂₂ of the central emitter E₂₂ is essentially no different from thelight output L₂₁, L₂₃ by the two outer emitters E₂₁, E₂₃. Therefore,even thought the central emitter E₂₂ is heated by its neighbours, theadjacent emitters E₂₁, E₂₃, these three emitters deliver a favourablyuniform light output pattern.

FIGS. 8-10 respectively show current pulses I_(11_pa), . . . I_(33_pa),relative temperature development T_(11_pa), . . . T_(33_pa) and relativelight output L_(11_pa), . . . L_(33_pa) when all nine emitters E₁₁, . .. E₃₃ of the array in FIG. 1 are simultaneously switched using the priorart approach. The drawings show that, in response to nine identicalcurrent pulses I_(11_pa), . . . I_(33_pa), the temperature of thecentral emitter E₂₂ is highest, and lowest at the four outer corneremitters E₁₁, E₁₃, E₃₁, E₃₃. As a result, the light output of the arrayis uneven, with the hotter emitters E₁₂, E₂₁, E₂₂, E₂₃, E₃₂ deliveringthe lowest light output L_(12_pa), L_(21_pa), L_(22_pa), L_(23_pa),L_(32_pa).

FIGS. 11-13 respectively show current pulses I₁₁, . . . I₃₃, temperaturedevelopment T₁₁, . . . T₃₃ and light output L₁₁, . . . L₃₃ when all nineemitters E₁₁, . . . E₃ of the array in FIG. 1 are simultaneouslyswitched using the inventive method. The drawings show that, in responseto nine tailored current pulses I₁₁, . . . I₃₃, the light output of thearray is even or homogenous, with all emitters E₁₁, . . . E₃ deliveringessentially the same light output levels L₁₁, . . . L₃₃, even though the“inner” emitters are heated by their neighbours. By taking these heatingeffects into account when computing the current shapes, a favourablehomogenous light output is obtained.

As explained above, the pulse history of an emitter will determine itsbehaviour during a subsequent pulse, i.e. any preceding pulse may affectthe behaviour of an emitter if the preceding pulse was applied to thatemitter or to a neighbouring emitter, and if the temperature of any ofthose emitters is still greater than its steady state or ambient value.

FIG. 14 shows how the inventive method can take pulse history intoconsideration when all nine emitters E₁₁, E₁₂, . . . , E₃₃ are to bedriven in a pulse event following the pulse event of FIG. 11. Thecurrent profile computation module takes into consideration thetemperature of each emitter E₁₁, E₁₂, . . . , E₃₃ prior to the intendedpulse event interval, as shown in FIG. 12, and computes the necessarycurrent profile shapes accordingly. A set of nine such tailored currentpulses I₁₁, I₁₂, . . . , I₃₃ is shown in FIG. 14. The relative lightoutput L₁₁, L₁₂, . . . L₃₃ of the entire emitter array is shown will bethe same as in FIG. 13, i.e. all nine emitters deliver the same lightoutput so that the emitter array delivers a favourably uniform lightoutput even though the emitters had different temperatures prior to thesecond pulse event of FIG. 14.

FIG. 15 shows light output as a function of emitter position in anarray, for example in the 3×3 matrix of FIG. 1, when current pulses donot take thermal crosstalk into consideration. The diagram shows lightoutput curves, each decreasing to different levels. Curve 150corresponds to an emitter heated by four surrounding emitters, forexample the central emitter E₂₂ in the matrix of FIG. 1. Curve 151corresponds to an emitter heated by three surrounding emitters, forexample emitters E₁₂, E₂₁, E₂₃, E₃₂ in the middle of each side of thearray in the matrix of FIG. 1. Curve 152 corresponds to an emitterheated by two surrounding emitters, for example emitter E₁₁, E₁₃, E₃₁,E₃₃ at an outer corner of the array in the matrix of FIG. 1. Curve 153corresponds to an emitter that is not heated by any surrounding emitter.

FIG. 16 shows an embodiment of the inventive LED arrangement 10, showingan emitter array 1 and a driver 2. The driver 2 is configured to applyindividual tailored current pulses 20 to the emitters as explained inFIGS. 5, 11 and 14. The driver 2 can be connected via a suitable bus todeliver the current pulses 20 to the emitters of the array 1. For a 2×2array, the driver 2 will be realised to generate four current pulses 20;for a 3×3 array, the driver 2 will be realised to generate nine currentpulses 20, etc. In this exemplary embodiment, a thermal environmentmodule 3 applies a suitable model that is given the thermal timeconstants τ of the emitters, and which predicts emitter temperaturebehaviour following a current pulse. In this embodiment, the thermalenvironment module 3 uses one thermal time constant for each emitter,for example. The thermal time constants τ may be stored in a memory. Thethermal environment module 3 provides a current profile computationmodule 4 with information 30 necessary to determine the current pulseprofiles 40 of the tailored current pulses 20 that will be required toachieve a desired light output pattern. Such information can compriseinput temperature data, output current shape data, etc. Here also, thecurrent profile computation module will be realised to generate fourcurrent pulse profiles 40 for a 2×2 array, nine current pulse profiles40 for a 3×3 array, etc.

A further circuit may be used to obtain information about the actualtemperature of the emitters. For example, an initial condition for thethermal environment profile can be established by measuring the forwardvoltages of the emitters and estimating the actual temperatures. To thisend, this embodiment of the inventive LED arrangement 10 also comprisesa forward voltage measuring module 5, and the measured forward voltages50 can be passed to the thermal environment module 3 which uses them topredict the temperature behaviour of the emitters.

FIG. 17 shows a an embodiment of the inventive LED arrangement 10 fordriving two different colour arrays 1A, 1B, for example for a dualcolour flash. The diagram shows a specification source 160 thatspecifies the desired light on scene in terms of lumen and colourtemperature. Information from an LED colour table 161 and an LEDtemperature table 162 is fed to a behaviour model 163, which computes aset of basic current pulse shapes IA, IB for the emitter arrays 1A, 1B.In temperature crosstalk correction modules 164A, 164B, the effect of ahot emitter neighbour is factored into each current pulse, and correctedcurrent pulse shapes IA′, IB′ are forwarded to the matrix drivers 2A, 2Bwhich apply the corrected current pulses (e.g. as shown in FIGS. 5, 11,14) to the emitters. The result is a homogenous light output L on ascene with a desired colour temperature, even though the emitters heatup in response to current pulses. A product based on the embodimentshown in FIG. 17 will be characterized by very advanced control onaccount of the feedback mechanism, and may be relatively expensive.

FIG. 18 shows a more economical realisation. Here, a simplerfeed-forward system is used, in which the emitters are divided into twogroups 1A, 1B. In this embodiment, a split current module 165 deploys asplit current algorithm that combines information from an LED colourtable 161 with information from a specification source 160 to divide theavailable current into current pulse sets IA, IB for the two emittergroups 1A, 1B. The result is a relatively homogenous light output L on ascene. The LED colour table 161 can be compiled using informationprovided by a thermal environment module such as that described in FIG.16. With a suitable LED colour table 161, the ratio of a first emittercolour to a second emitter colour during a current pulse can bedetermined, for example to adjust the ratio of warm white to cool whiteduring a pulse sequence in order to achieve a uniform colour temperaturedistribution in the scene.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention as described by theappended claims.

For the sake of clarity, it is to be understood that the use of “a” or“an” throughout this application does not exclude a plurality, and“comprising” does not exclude other steps or elements. The mention of a“unit” or a “module” does not preclude the use of more than one unit ormodule.

What is claimed is:
 1. A method of driving an emitter array, the methodcomprising: determining a thermal environment profile for a plurality ofemitters of the emitter array; computing a current pulse profile for atleast one of the plurality of emitters based on the thermal environmentprofile; and applying a current pulse with the computed current pulseprofile to the at least one of plurality of emitters.
 2. The methodaccording to claim 1, wherein the thermal environment profile comprisesinformation relating to temperature development in the plurality ofemitters of the emitter array.
 3. The method according to claim 1,wherein the thermal environment profile comprises information relatingto pulse histories of the plurality of emitters of the emitter array. 4.The method according to claim 1, wherein the determining the thermalenvironment profile comprises considering at least one of temperaturedevelopment and pulse history over an interval that exceeds a durationof a current pulse interval.
 5. The method according to claim 1, whereinthe determining the thermal environment profile of the emitter arraycomprises using a thermal predictive model.
 6. The method according toclaim 5, wherein the thermal predictive model implements thermal timeconstants of individual emitters of the plurality of emitters.
 7. Themethod according to claim 5, wherein the thermal predictive modelconsiders temperature dependency of a desired colour point.
 8. Themethod according to claim 1, wherein the computing the current pulseprofile for an individual emitter of the plurality of emitters furthercomprises computing the current pulse profile based on a light outputparameter that comprises at least one of a colour, a required lightintensity of the individual emitter and a number of neighbouringemitters.
 9. The method according to claim 1, wherein the computing thecurrent pulse profiles comprises computing the current pulse profile forat least a first emitter and a second emitter of the plurality ofemitters to adjust a ratio of a colour of the first emitter to a colourof the second emitter during a current pulse.
 10. The method accordingto claim 1, wherein the current pulse applied to the at least one of theplurality of emitters has a shape that is based on the current pulseprofile.
 11. A device comprising: an array of emitters; a thermalenvironment module configured to determine a thermal environment profileof the emitter array; a current profile computation module configured tocompute a current pulse profile for an emitter of the emitter arraybased on the thermal environment profile; and a driver configured toapply a current pulse with the computed current pulse profile to theemitter.
 12. The device according to claim 11, wherein a surface area ofindividual emitters in the array comprises at most 0.25 mm².
 13. Thedevice according to claim 11, wherein the emitter array comprises atleast four emitters.
 14. The device according to claim 11, wherein theemitter array comprises at least nine emitters.
 15. The device accordingto claim 11, wherein the emitter array comprises at least one cool whiteemitter and at least one warm white emitter.
 16. The device according toclaim 11, wherein the device is a camera flash module.
 17. The deviceaccording to claim 11, wherein the device is an automotive lightingmodule.
 18. The device according to claim 11, wherein the driver isfurther configured to apply the current pulse having a pulse shape thatis based on the current pulse profile.
 19. The device according to claim18, wherein the current pulse applied to at least two different emittersin the array has a different pulse shape.
 20. The device according toclaim 11, wherein the pulse shape reflects an increase in current over atime period of the current pulse.